Developement of BAC mutagenesis has opened a new avenue for the manipulation of large virus genomes, including Herpesviridae. Stability, relatively simple manipulation and large insert size of BAC constructs within the Escherichia Coli has marked a turning point enabling various modifications like point mutations, insertions and deletions of viral or foreign genes. To better understand the replication of HSV-1 and its spread during infection, in silico and with molecular cloning methods, we constructed a recombinant virus in which EGFP (enhanced green fluorescent protein) expression cassette was inserted into the intergenic region between sequences UL3 and UL4. With „En passant“ mutagenesis, a simple and efficient lambda Red recombination technique, we have generated HSV-1 LD-pINS i HSV-1 LD-EGFP recombinant viruses in two key steps. In the first step, linearized recombination cassettes pInserter-MCS and pInserter-EGFP were inserted via Red recombination into target site using kanamycin positive selection (BAC LD-pIns-KAN, BAC LD-EGFP-KAN). In the second step, I-SceI endonuclease cleaved at its recognition site and created a double strand brakes, DSB. The adjoining duplicated sequences, present at the ends of dsDNA, served as the new substrate for second Red recombination, resulting in loss of kanamycin (BAC LD-pIns, BAC LD-EGFP). Successful integration of EGFP expression cassette into HSV-1 genome was confirmed by fluorescent microscopy and western blot. HSV-1 LD-EGFP infected cells even without promoter presence emit sufficient level of fluorescence, necessary for monitoring of viral replication in vitro. Moving away from the center of viral plaque where EGFP fluorescence of infected cells is expressed the most, fluorescence drops significantly. With this study we established a simple and direct system of continuous monitoring of HSV-1 infection in vitro, and opened the possibility of its application to other HSV-1 strains. Far efforts of our laboratory will be directed to adaptation of vector pInserter-MCS for the application of eg. miRNA sponge technology that would clarify the role of miRNA in virulence.